In a conventional telephone network, a subscriber who wants to make a call to another subscriber normally dials the called subscriber's number. This number corresponds to the network address of the called subscriber's terminal in the telephone network and thus identifies directly a subscriber terminal of the network. During call establishment, the number is carried in the call request and is used in the switching centers of the telephone network directly for routing purposes and to locate the called subscriber.
This mode of addressing a subscriber in a communications network has disadvantages if the subscriber has two or more subscriber terminals. To be able to reach the called subscriber, the calling subscriber must know the numbers of all these terminals.
Concepts are known which permit addressing directed to the subscriber rather than the terminal, and thus overcome this disadvantage. Such a concept is described, for example, in an article by Jonas Sundborg, "Universal Personal Telecommunication (UPT) --Concept and Standardisation", Ericsson Review No. 4, 1993, pages 140 to 154.
In that concept, each subscriber is identified by a unique, network-transparent UPT number. The numbering scheme for the UPT numbers is standardized in E.168, the numbering structure being based on E.164, which is the recommendation for the ISDN numbering plan. The UPT number consists of three parts: a country code, a UPT service provider indicator, and a subscriber number.
During call setup, the UPT number is routed through the communications network to the UPT service provider, and the latter then determines that terminal via which the subscriber can be reached at the moment. The physical network address assigned to this terminal is determined and is then used to establish the call to the called subscriber. This network address is usually the number of the subscriber terminal determined.
With all these known methods of addressing a subscriber in a communications network, however, the problem arises that the calling subscriber must know a number specifically intended for this manner of communication with the called subscriber.
This problem will be aggravated as the number of means of communication (telephony, E-mail, address in different data networks) increases.